The present invention relates to offset machines and more particularly to inking and wetting methods of the offset plate.
Before referring to the problems which the invention intends to solve, the operation of an offset machine will be briefly described.
FIG. 1 shows a simplified exemplary mechanism of an offset machine. A printing cylinder 1, on which an offset plate 2 is mounted, first contacts wetting rollers 3a and 3b then inking rollers 4a and 4b. The ink on plate 2 is then transferred onto a so called blanket or cylinder 5. Blanket 5 contacts a pressing cylinder 6. A sheet of paper 7 passing between cylinders 5 and 6 is printed by the ink present on blanket 5.
The surface of the offset plate 2 is so constituted that, during the inking phase, the ink deposits only on the desired areas of the plate for constituting an image.
FIG. 2 is an enlargement of a cross section of the offset plate 2. A resin film has been deposited on the rough surface 20 of plate 2 then etched as a function of the image to be reproduced while leaving resin dots 21 having variable dimensions and resin free regions 22. The term "density" will be referred to hereafter as the ratio between the surface area of the dots 21 and the surface area of the regions 22. During wetting by rollers 3, the jaggedness of surface 20 collects the water which is moreover repelled by the dots 21 which are hydrophobic. Then, during inking by rollers 4, the ink which is greasy and therefore hydrophobic deposits essentially on dots 21 which are moreover oil attracting.
According to the respective amounts of water and ink deposited on plate 2, the ink coverage ratio changes, that is, the ink more or less covers the dots 21, while the water occupies the remaining surface. Thus, the problem encountered is to depart as little as possible from the water-ink equilibrium which corresponds to the ideal state where the ink would exactly cover the dots 21, in other words, where the coverage ratio would be equal to the density.
For a proper operation, the respective amounts of water and ink have to range within determined limits, namely an upper limit where the ink would smudge when transferred onto blanket 5 and a lower limit where the water in regions 22 would not entirely cover the peaks of the jagged surface 20. If the lower limit is not observed, ink also sticks on regions 22 and it is necessary to clean the offset plate.
In conventional offset machines, the water ink equilibrium is obtained by separate manual adjusting of the wetting and inking. The effect of these adjustments is liable to be controlled by a control bar which is printed simultaneously with each image. The control bars comprise several screened areas having different dot sizes and different ideal coverage ratios. These control bars are viewed or examined with optical densitometers on the printed sheet to determine whether wetting and inking adjustments are to be readjusted.
French patent application FR A-2,556,283 (under priority of U.S. patent applications Ser. No. 560,837 of Dec. 13, 1983 and No. 618,252 of Jun. 7, 1984) describes a system for measuring the average amount of water present on the inking roller in motion. The roller is illuminated under a non-normal incidence and the reflected light is collected by several optical sensors. The signals provided by these sensors are then processed for obtaining an indication of the average amount of water on the roller. A drawback of this system is that no indication on the coverage ratio of the offset plate is obtained, which, as seen above, is essential for detecting the water-ink equilibrium. A further drawback is related to the illumination of the roller under an incidence different from the normal, which causes intensity variations at the sensors depending on the surface defects and roller misalignment.